Fragments of the finger bone were recovered from Denisova cave in the Altai mountain range that straddles Russia, Mongolia, China and Kazakhstan. The cave was occupied by humans for 125,000 years and a variety of stone tools and bones have been recovered.

The size of the bone has led scientists to believe it came from a child, aged between five and seven, though they are unable to say whether it was male or female.

Scientists at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, ran genetic tests on the bone fragments and were stunned to find it did not match the DNA profile of Neanderthals or early modern humans.Johannes Krause sequenced DNA from mitochondria, the sub-cellular bodies that carry genetic material passed down only the maternal line. Because the DNA came from the mother, they called the creature "X-woman".

"It really looked like something I had never seen before. It was a sequence which is similar in some ways to humans, but still quite distinct," Krause said. It is the first time a new type of human has been identified from DNA alone.

By comparing the DNA with sequences from Neanderthals and modern humans, Krause's team concluded that modern humans shared a common ancestor with the creature a million years ago. Humans and Neanderthals diverged from an ancestor that lived 500,000 years ago.

When Krause saw the results of the genetic test, he called project leader Svante Pääbo. "It was absolutely amazing, I didn't believe him. I thought he was pulling my leg," Pääbo said. The bone fragments were recovered from a layer of rock in the cave dated to between 48,000 and 30,000 years ago.

The first humans to move from Africa to Eurasia were Homo erectus 1.9m years ago, but scientists believed they died out around 100,000 years ago. The new species probably migrated from Africa more recently, around 1m years ago, and survived in Eurasia until at least 40,000 years ago.

Krause's team is now analysing DNA from the nuclei of cells in the finger fragments in the hope of locating the species in the human family tree. The tests should also indicate whether there was any interbreeding between the new species, Neanderthals and modern humans.

"There were at least three different forms of humans in the area between 30,000 and 40,000 years ago, and there were also the hobbits in Indonesia, so the picture of what was around in human form in the late Pleistocene gets a lot more complex and a lot more interesting," Pääbo said.

Chris Stringer, head of human origins at the Natural History Museum in London, said: "This new DNA work provides an entirely new way of looking at the still poorly understood evolution of humans in central and eastern Asia."

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